The disclosures herein relate generally to electronic circuitry, and more particularly to an optical receiver chain for components of a photoplethysmograph signal.
A plethysmograph is a device that measures changes in volume, typically changes in the volume of blood or the volume of air within a tested portion of a human body. A photoplethysmograph (“PPG”) is a plethysmograph that uses a beam of light to detect changes in the volume. A pulse oximeter is a type of PPG that measures changes in the volume of arterial blood within the tested portion of the body to determine an oxygen saturation level (SpO2). The change can be measured with each heart-beat.
In pulse-oximetry/optical heart rate monitoring (“HRM”), an optical transmitter uses one or more light-emitting diodes (“LEDs”) to emit light onto a body, where changes in arterial blood volume modulate the intensity of the LED light. An optical receiver receives the modulated LED light (at single or multiple wavelengths) via a photo detector, such as a photodiode/photo-transistor or avalanche photodiode (“APD”), and generates a pleth signal in response to the modulated LED light. The oxygen saturation level (SpO2) can then be obtained from the pleth signal. In at least one example, the LED is pulsed to save power.
Pulse oximeters can operate as transmission-type devices or reflectance-type devices. With transmission-type devices, the LED light passes through the body before the LED light is received by the photo detector. With reflectance-type devices, the LED light reflects off of the body before the LED light is received by the photo detector.
FIGS. 1A-1B are diagrams of a conventional transmission-type pulse oximeter 100. As shown in FIGS. 1A-1B, pulse oximeter 100 includes an LED 110 that generates pulsed light 112, an LED 114 that generates pulsed light 116, and a photo detector 118 that detects the pulsed lights 112 and 116. LED 110 generates the pulsed light 112 having a first frequency, such as infrared (IR), while LED 114 generates the pulsed light 116 having a second frequency, such as red (R). Also, conventional LEDs, such as LED 110 and LED 114, also generate a minor amount of off-axis light, which is scattered and detected by photo detector 118.
As shown in FIG. 1A, the LEDs 110/114 and photo detector 118, which are connected to a processing chip 120, are spaced apart and positioned to face each other. As shown in FIG. 1B, as a transmission-type device, the pulsed lights 112 and 116 pass through a tested portion of a human body, such as a finger 122, before being received by photo detector 118.
FIGS. 2A-2B are diagrams of a conventional reflectance-type pulse oximeter 200. As shown in FIGS. 2A-2B, as with pulse oximeter 100, pulse oximeter 200 includes an LED 210 that generates pulsed light 212, an LED 214 that generates pulsed light 216, and a photo detector 218 that detects the pulsed lights 212 and 216. LED 210 generates the pulsed light 212 having a first frequency, such as infrared (IR), while LED 214 generates the pulsed light 216 having a second frequency, such as red (R). Also, LED 210 and LED 214 generate a minor amount of off-axis light, which is scattered and detected by photo detector 218.
As shown in FIG. 2A, the LEDs 210/214 and photo detector 214, which are connected to a processing chip 220, lie adjacent to each other and face in the same direction. As shown in FIG. 2B, as a reflectance-type device, the pulsed lights 212 and 216 reflect off of a tested portion of the body, such as a finger 222, before being received by photo detector 218.
The pleth signal generated by a pulse oximeter has an ambient DC component, a pleth DC component, and a pleth AC component that rides on the pleth DC component. The ambient DC component results from ambient conditions. Non-hospital environments (such as wearable fitness trackers and mobile patient monitoring bands) are difficult to control, so many factors can affect the ambient DC component of the pleth signal. The ambient DC component (due to biological drifts or environmental variations) is exacerbated in wearable/portable applications (e.g., fitness tracking), where ambient conditions are not well-controlled, such as: (a) sudden shift from sunlight to shade; and (b) random or uncontrolled motion (e.g., biking and running).
FIG. 3 is a diagram of a conventional pulse oximeter system 300. As shown in FIG. 3, pulse oximeter system 300 includes an optical transmitter 310 that generates pulsed light 312. Optical transmitter 310 includes a LED 314, a current source 316 that sinks current from LED 314, and a switch 318 that opens and closes to generate the pulsed light 312.
System 300 also includes a channel 320, which includes all of the conditions that can affect the pulsed light 312. Changes in the arterial blood volume within the tested portion of a human body can be represented by a body signal generator 322 that outputs a body signal BS, while the interaction of the body with the pulsed light 312 can be represented by a modulator 324 that amplitude modulates the pulsed light 312 with the body signal BS to generate a pulsed body-modified light 326. Also, the interaction of the body with the body-modified light 326 dims the body-modified light 326, and can be represented by an attenuater 330 that reduces the intensity of the pulsed body-modified light 326 to produce a pulsed attenuated body-modified light 332.
Further, the ambient environmental conditions can be represented by an ambient signal generator 334 that outputs an ambient DC voltage VA, while the interaction of the environmental conditions with the attenuated body-modified light 332 can be represented by an adder 336 that adds the ambient DC voltage VA to the attenuated body-modified light 332 to form a pulsed modulated light 338, which has a DC offset due to the ambient DC voltage VA.
System 300 additionally includes an optical receiver 340 that receives the pulsed modulated light 338, and generates a sampled pleth signal SS in response to the pulsed modulated light 338. Receiver 340 includes a photo detector 342 that generates a photo current IP in response to the pulsed modulated light 338, and a transimpedance amplifier 344 that converts the photo current IP into a voltage VP.
Receiver 340 also includes a switch 346 that opens and closes, and a resistor/capacitor combination 348 that samples and holds the voltage VP when switch 346 is closed and opened to generate the sampled pleth signal SS. Switch 346 is closed during all or part of the time that switch 318 is closed. An analog-to-digital converter (“ADC”) then digitizes the sampled pleth signal SS.
FIG. 4 is a diagram of a conventional pleth signal PL. As shown in FIG. 4, the pleth signal PL has an ambient DC component 410, a pleth DC component 412, and a pleth AC component 414. The ambient DC component 410 is a pedestal or baseline, which exists even when the LED is turned OFF. The ambient DC component 410 can be caused by varying ambient illumination conditions or biological processes, and it may vary based on motion artifacts.
The pleth DC component 412 results from variations in light absorption by the structures within the tested portion of the body. Some structures within the body, such as the skin and nonpulsatile blood, absorb a constant amount of light, which produces the pleth DC component 412, while the pulsating arterial blood flow absorbs a variable amount of light, which produces the pleth AC component 414.
The pleth AC component 414 rides over the pleth DC component 412, which is proportional to applied LED illumination. In the HRM example, the useful signal for HRM is obtained from the pleth AC component 414 that rides over the large DC signal, which includes an ambient DC component 410 and a pleth DC component 412. The pleth AC component 414 (from which the oxygen saturation level (SpO2) or heart rate information is obtained) is proportional to the pleth DC component 412. The ratio of the AC to DC component is referred to as the Perfusion Index.
The pleth AC component 414 is relatively small, but can be increased by increasing the transmit intensity of the light output by LED 314. Increasing the transmit intensity of the light output by LED 314 increases the magnitude of the pleth AC component 414 more than the magnitude of the pleth DC component 412. However, only small increases can typically be made without saturating the ADC that digitizes the sampled pleth signal SS.
The ambient DC component and the pleth DC component consume a significant portion of the ADC's dynamic range for PPG measurement and is considered an “interferer.” If the ADC is designed with a wide dynamic range to accommodate the signal plus all possible sources of DC offset, then it would be energy inefficient and unsuitable for portable applications.
Some techniques use a single point for ambient cancellation after a front-end transimpedance amplifier (“TIA”). But those techniques are insufficient, because the signal is lost if the TIA saturates. Additional techniques apply a single DC offset correction point at the TIA's input. But those techniques incur a noise penalty in correcting DC across a wide range (e.g., ambient level plus pleth DC level). Such noise penalty imposes stringent requirements on an input current digital-to-analog converter (“DAC”). Other techniques correct DC level by changing the LED level. A lower LED current level decreases the pleth DC level. However, while those techniques directly impact the pleth level, so they can affect the AC component itself, because the AC component is proportional to the DC component, thereby reducing the receiver signal-to-noise ratio (SNR).